1. Field of the Invention
The present invention relates to microchip heaters for microfluidic and micromechanical applications, such as integrated chemical microreactors for decomposition and detection of bioorganic compounds such as DNA, ink-jet printer heaters for firing ink for printing purposes, optical switching based on vapor bubble formation to deflect a light beam, and optical switching based on liquid crystals.
2. Description of the Related Art
As is known, some fluids are processed at temperatures that should be regulated in an increasingly more accurate way, for example, when chemical or biochemical reactions are involved. In such situations, there is often also a need to use very small quantities of fluid, owing to the cost of the fluid, or to low availability.
This is the case, for example, of the DNA amplification process (PCR, i.e., Polymerase Chain Reaction process), wherein accurate temperature control in the various steps (repeated pre-determined thermal cycles are carried out), the need to avoid as far as possible thermal gradients where fluids react (to obtain here a uniform temperature), and also reduction of the used fluid (which is very costly), are of importance in obtaining good reaction efficiency, or even to make reaction successful. Microchip heaters are particularly suited for this application.
Other examples of fluid processing with the above-described characteristics are associated for example with implementation of chemical and/or pharmacological analyses, and biological examinations. Other situations that require a miniaturized heater that is one or more of more accurate, quicker reacting, more durable, longer-lived, more controllable and less expensive to manufacture include ink-jet printers heaters and optical switching heaters to name a few.
At present, various techniques allow thermal control of chemical or biochemical reagents. In particular, from the end of the 1980s, miniaturized devices were developed, and thus had a reduced thermal mass, which could reduce the times necessary to complete the DNA amplification process. Recently, monolithic integrated devices of semiconductor material have been proposed, able to process small fluid quantities with a controlled reaction, and at a low cost (see, for example, U.S. patent application Ser. No. 09/779,980, filed on Feb. 8, 2001; Ser. No. 09/874,382 filed on Jun. 4, 2001; and Ser. No. 09/965,128, now issued U.S. Pat. No. 6,710,311; all assigned to STMicroelectronics, S.r.l. and incorporated herein by reference).
These devices comprise a semiconductor material body accommodating buried channels that are connected, via an input trench and an output trench, to an input reservoir and an output reservoir, respectively, to which the fluid to be processed is supplied, and from which the fluid is collected at the end of the reaction. Above the buried channels, heating elements and thermal sensors are provided to control the thermal conditions of the reaction (which generally requires different temperature cycles, with accurate control of the latter), and, in the output reservoir, detection electrodes are provided for examining the reacted fluid. The heat is generated by supplying electric current to a metal heating element formed on a wafer comprising a semiconductor body with contact regions in electrical contact with the two opposite ends of the heating element and connected to a drive transistor, typically a MOSFET formed on the same wafer.
Current inkjet technology relies on placing a small amount of ink within an ink chamber, rapidly heating the ink and ejecting it to provide an ink drop at a selected location on an adjacent surface, such as a sheet of paper. Traditionally, ohmic resistors which heat up rapidly when current is passed therethrough have been used to provide the necessary temperature increase of the ink. See, for example, a detailed discussion of ink ejection in an article titled “Thermodynamics and Hydrodynamics of Thermal Ink Jets,” by Allen et al., Hewlett-Packard Journal, May 1985, pp. 20-27, incorporated herein by reference.
Microchips are highly suited for miniaturized heater applications. Generally, present techniques for generating local heating in a microchip are based on ohmic resistors made of metal alloys, such as TaA1, HfB, ternary alloys, etc., or polycrystalline semiconductors. The heating resistor is driven by external circuitry or an integrated power MOSFET. In existing applications, such as thermal ink-jet printers, the heating resistor value is preferably higher than the MOSFET channel resistance (RON or RDS) to minimize the parasitic effects and dissipate power in the heating resistor only. Normally, each power MOSFET occupies a large percentage of the chip area to minimize its RON.
One drawback with this arrangement is that the resistance of such ohmic resistors is fixed and cannot be modulated, thus limiting their flexibility. Other drawbacks are that ohmic resistors are subject to material degradation (such as oxidation, segregation, etc.), and electromigration, especially at high temperatures. These phenomena limit their lifetime and are a concern for the reliability of devices that incorporate them into their design.
Yet another drawback is power control. Ohmic resistors, which are either current or voltage driven, dissipate a power that is a quadratic function of the parameters. This results in poor control over their output, as small variations in current or voltage can cause significant fluctuations in power and temperature output.
The present invention provides a miniaturized heater that provides the desired characteristics for many microfluidic and micromechanical applications, while overcoming the drawbacks noted above, while providing other related advantages.